An electrostatic sensor is used to measure electrostatic charge, primarily by measuring the effect, on an electrode, of lines of flux emulating from the charge being measured. The sensing electrode is associated with an earth shield and provides an output which depends on the magnitude of the charge being sensed. More particularly, the sensor output will usually be proportional to the magnitude of the charge in front of the sensing surface of the electrode, measured as a weighted average across the area of the surface. The degree of sensitivity is also related to the area of the sensing surface and the proximity of the earth shield. The field of view to which the surface responds can be modified, for example by modifying the shape of the earth shield or by moving the sensor toward or away from the object of interest. A measurement made by the sensor at a particular point in time will represent the view of the sensor at that point in time. In the case of moving objects, a consequence of such measurements can be used to provide a more complete understanding of the system being observed, but this approach can give rise to problems, particularly with fast moving or quickly changing system. It will be appreciated that electrostatic sensors can be utilised with regard to monitoring machinery such as bearing assemblies for wear or to provide images for medical analysis such as with regard to the skin of a person.
It is known to isolate the earth shield and provide a bias voltage or charge in order to view particular charge masses.
An example of a prior electrostatic sensor is provided in International Patent Application No. PCT/GB2003/005559 in the name of Rolls Royce plc. It will be appreciated that one advantage with regard to electrostatic imaging with respect to locating defects in the skin of a person is that the image produced is not purely of the surface but has some depth. However, as indicated above the nature of electrostatic sensing leads to distortion. Thus the images produced generally have no tracer elements and therefore render it difficult to determine the exact position of features when not viewed in real time. It is known to provide surface images by appropriate photography utilising a camera such as a charge coupled device in order to create a surface image of the defect. However, such surface images require the skill of a practitioner in order to fully interpret the image received. With regard to cancerous defects, it will be understood that it is necessary to remove deleterious tissue but removal of perfectly good tissue should be avoided. It will be appreciated that generally with regard to skin carcinoma the tissue to be removed will be visible and therefore consideration of cosmetic effects has to be taken into account. Ideally, scaring should be minimised subject to removal of all the deleterious tissue.